1. Field of the Invention
The present invention relates to the testing of feed-throughs for cryogenic and ambient service applications, and more particularly to a test rig for testing such feed-throughs under cryogenic temperature conditions.
2. Description of Related Art
Numerous applications require the use, storage and delivery of cryogenic liquids, including liquid hydrogen (LH2), liquid oxygen (LOX) and liquid nitrogen (LN2) to name a few, as well as the corresponding gases at cryogenic temperatures. These fluids are used as propellants, coolants and reagents in a wide variety of space-based and terrestrial applications. It is also often preferable to store certain normally gaseous (at standard temperature and pressure—“STP”) fluids, e.g. those mentioned above, in the densified liquid state even if they are to be used in the gaseous state. This is because greater mass can be stored in a given volume in a liquid state compared to the gaseous state of the same fluid. Liquid-state storage and delivery/use of these fluids requires maintaining them at cryogenic temperatures. To measure and collect data as to the status of a cryogenic fluid (liquid or gaseous) as well as other system parameters, probes and other instrumentation often must contact or be provided in proximity to the cryogenic temperatures and fluids within the vessel or piping where it is stored or flowing. This means either the probe or instrumentation wires must traverse the cryogenic boundary to deliver instrumentation signals to a computer or other processing device on the outside. In a closed system, such probes typically are provided within the storage vessel (or piping) via feed-throughs disposed in the vessel (or piping) wall. A challenge in this area with respect to cryogenic fluids is the operating temperatures used, which are below 100K and can be as low as a few degrees Kelvin. To be effective, a cryogenic feed-through must not only be capable to withstand cryogenic operating temperatures, but it also must provide an effective seal at such temperatures under the prevailing pressure conditions.
As known in the field, a feed-through is a fitting that has a port or opening through which probes, transducers, their wires or other leads are allowed to penetrate the vessel wall (or vacuum jacket or piping), while providing a seal around the penetrating elements to protect the integrity (e.g. maintain pressure, inhibit leakage) of the vessel contents. Feed-throughs are commonly used in many fields including aeronautics, aerospace, biomedical, compressed gas, and many more. Better feed-throughs can improve data quality, data integrity, and the ease of installation. New feed-throughs can also increase sensor response time if sensors are mounted bare instead of being housed in sheaths that pass through the pressure boundary, such as swaged thermocouple sheaths. With a properly designed feed-through, probes and other instrumentation may be fed directly into a storage vessel or through another material boundary and be plugged directly into an electronics box, without the need for extra wires or solder connections inside the vessel or on the fluid side of that boundary. This will result in a significant reduction in complexity and cost, both in labor and materials, in many applications.
In the case of a cryogenic feed-through, which accommodates instrumentation or their wires for cryogenic fluids, the feed-through must be capable of withstanding the pressure gradients and cryogenic temperatures to which it will be subjected during actual use while maintaining an appropriate seal. As new cryogenic feed-throughs are developed to address the concerns in the preceding paragraph, the best way to ensure that they will perform under and withstand the anticipated operating conditions is to test and proof them under the same conditions. Unfortunately, however, manufacturers typically do not appear to do this. Instead, cryogenic feed-throughs appear to be rated for specific operating pressures and temperatures based on design calculations, ambient-temperature pressure testing or some combination of both. It is believed that cryogenic feed-throughs are not typically tested under cryogenic-temperature conditions due to the dangers associated with handling liquid cryogens, particularly if a candidate feed-through were to catastrophically fail or be ejected from a vessel full of liquid cryogen during a test. Where feed-throughs might be tested under cryogenic conditions, such testing is cumbersome and can be dangerous for these reasons. Furthermore, many feed-through manufacturers may not have the capability to pressure test their products at cryogenic temperatures.
The National Aeronautics and Space Administration (NASA) has many cryogenic test facilities that operate under a variety of extreme temperatures and pressures, all requiring feed-throughs to accommodate cryogenic-fluid instrumentation. Very few feed-throughs have been tested at the temperature and pressure extremes required by these centers. There is a need in the art for a test rig that will safely permit testing of cryogenic feed-throughs under cryogenic-temperature conditions at anticipated operating pressure gradients. Such a test rig preferably will permit one to quantify a leak rate of cryogenic fluid through the feed-through under operating conditions, and to correlate the leak rate with other parameters such as the pressure gradient across the feed-through. Such a test rig would be valuable for developing new types of cryogenic feed-throughs, which would be able to be tested under anticipated actual conditions of pressure and cryogenic temperature.